1. Field of the Invention
The present invention relates to a process for preparing aromatic urethanes or aromatic carbamic acid esters. More particularly, this invention relates to a process for synthesizing aromatic urethanes from an aromatic nitro compound, an organic compound having a hydroxyl group and carbon monoxide in the presence of a catalyst.
2. Description of the Prior Art
Aromatic urethanes (hereinafter referred to simply as urethanes) have been heretofore produced generally by reaction of an aromatic isocyanate (hereinafter referred to as isocyanate for brevity) with an organic compound having hydroxyl groups (hereinafter referred to simply as hydroxyl group-containing compound). In recent years, production of isocyanates has become difficult due to shortages and rising costs of starting materials and also due to strong toxity of intermediate products. Under these circumstances, many novel methods for the production of urethanes have been investigated and developed. However, such methods have not been useful effectively on an industrial scale due to vital defects and problems to be solved.
For example, U.S. Pat. No. 3,338,956 describes a method in which urethanes are produced from an alcohol, carbon monoxide and an aromatic nitro compound (hereinafter referred to simply as nitro compound) in the presence of rhodium chlorocarbonyl as catalyst. In this method, however, the yield of the desired product is low even when the reaction is effected in the presence of a large amount of the catalyst over a long period of time. Therefore, it is not considered that such a method is economically productive of highly pure urethanes.
Further, German Pat. No. 1,543,051 teaches a method for the preparation of urethanes in which a hydroxyl group-containing compound, carbon monoxide and a nitro compound are interacted in the presence of a catalyst of a carbonyl group-containing derivative of a metal of Group VIII of the Periodic Table of Elements in coexistence with a promoter composed of a salt of a metal selected from metals which are capable of existing in two or more valence states. However, this method is not useful on an industrial scale of production since the yield of product is low even when a mononitro compound is employed as starting material, and use of a dinitro compound results in an even lower yield.
Moreover, there has been proposed a method using palladium and a Lewis acid as catalyst (U.S. Pat. No. 3,531,512). By this method, urethanes are obtainable at yields as high as 80%-90% under certain conditions even when a dinitro compound is employed as starting material. In order to attain such high yields, however, the reaction must be effected under severe conditions such as a carbon monoxide initial pressure of 190-350 kg/cm.sup.2 and a reaction temperature of 190.degree.-200.degree. C. Additionally, the method has an industrially vital disadvantage in that the Lewis acid, e.g., ferric chloride, used as promoter has a strong corrosive action against metal materials such as iron, stainless steel, etc. Accordingly, it is essential to use a glass or tantalum reactor in order to put this method into practice. The use of a glass or tantalum reactor under the above-described high temperature and pressure conditions presents several technical and economical problems.
Further, there is also known a method using as a catalytic system selenium, sulphur or compounds thereof, and a base or water (French Pat. No. 2,197,862). The reaction conditions of this method are found to be rather mild among the known or proposed methods. The French patent teaches that the bases usable in the method are aliphatic, aromatic and heterocyclic amines, and metal salts of carboxylic acids, sulfonic acids and phosphonic acids. The useful amines are found to be, for example, triethylamine, pyridine, quinoline, N,N-dimethylaniline, diethylamine, tertiary butylamine, 1,4-diazabicyclo(2,2,2)octane, N,N,N',N'-tetramethylethylenediamine, tetramethylenediamine and ethylenediamine. However, in order to satisfactorily induce the reaction in the presence of these amines, it appears necessary to use the amines in a fairly large amount with respect to the starting nitro compound. In fact, when dinitrotoluene is used as the nitro compound, the amine is employed in an amount equal to or greater than that of the dinitrotoluene. Experiments conducted by the present inventors revealed that when the amount of amine was reduced to about 1/6 that of dinitrotoluene, the reaction hardly proceeded, and that, with a reduction to about 2/3, the reaction was considerably slowed down, as will be described in detail in Comparative Examples 2 and 3 hereinafter. The use of such large amounts of amine involves many problems in economy and recovery operations. Further, the method inevitably involves formation of a certain amount of by-products, i.e., amino compounds, even if the reaction conditions are modified to some extent. Accordingly, the yield of urethane is 72%-73% at most. This method is unsuitable for attaining higher yields. As described hereinbefore, the French patent teaches the use of metal salts of carboxylic acids, sulfonic acids, or phosphonic acids as the base. However, the use of these metal salts is considered disadvantageous for the following reason. That is, when potassium acetate, for example, is used as the metal salt, an abnormal exothermic reaction is apt to take place, by which it becomes difficult to properly control the reaction temperature. The raising of the reaction temperature to above 180.degree. C. results in conversion of most of the reaction product to tar-like substances. The yield will be disadvantageously reduced to a considerable extent. In addition, the converted reaction solution emits a strong offensive odor similar to hydrogen sulfide and thus the urethane product obtained from the solution also has such offensive odor. The odor is difficult to remove. According to supplementary experiments conducted by the present inventors, such high yields as indicated in the examples of the French patent specification are difficult to obtain, with amino compounds being secondarily produced in fairly large amounts. Thus, the method of the French patent is not satisfactory for industrial urethane production. There are, accordingly, strong demands for development of more active catalysts and of improvement of yield.